1. Field of the Invention
The present invention relates to a photosensitive resin composition which is small in curing shrinkage and excellent in photosensitivity and a circuit board with metal support using the same, and a circuit board with metal support.
2. Description of the Related Art
In recent years, a material called a photosensitive resin composition has been used for a variety of applications such as a semiconductor buffer coat material, a surface protection material for a circuit board, an interlayer insulating material for a circuit board, a light waveguide, a color resist, a black resist, and an overcoat material for a color filter to be used in a liquid crystal display apparatus, a liquid crystal cell gap adjusting material, and a material for a flow path forming member of an ink-jet printer head.
In general, there are given, as main constituent materials for the above-mentioned photosensitive resin composition, a curable and crosslinkable monomer and polymer, and a photosensitizer, for example. A photo radical generator for generating a radical through photoirradiation or a photo acid generator for generating an acid through photoirradiation is used as the above-mentioned photosensitizer. The above-mentioned photo radical generator is mainly used as an initiator in the photocuring of a compound having an acryloyl group or a methacryloyl group, such as a multifunctional acrylic monomer. Although the photo radical generator provides a high curing speed, there are such drawbacks that curing is inhibited by oxygen in the air and curing shrinkage is large, for example. Meanwhile, the above-mentioned photo acid generator is used as an initiator in the photo curing of a multifunctional epoxy compound or a vinyl ether-based compound. Although a curable system using the photo acid generator is not inhibited by oxygen, the photo acid generator is difficult to be used in some applications because an acid remaining in a material system causes the corrosion of a metal material, for example.
From the viewpoint of solving such problems, a photocurable system using a photo base generator for generating a basic substance such as an amine through photoirradiation has attracted attention in recent years. The above-mentioned curable system using a photo base generator has such advantages that the system is hardly inhibited by oxygen and the photo base generator remaining in a material system is less liable to cause the corrosion of a metal material, for example. However, under the current circumstances, the curable system has such problems that the system is low in photosensitivity and the photo base generator itself is poor in heat resistance, for example.
A 1,4-dihydropyridine derivative, which is relatively high in photosensitivity and heat resistance, may be used as the above-mentioned photo base generator for the above-mentioned applications. For example, some photosensitive resin compositions each including a polyamide acid as a polyimide precursor blended with the above-mentioned 1,4-dihydropyridine derivative have already been proposed (see JP-A-H05-281717, JP-A-H07-134417, JP-A-H07-271034, JP-A-H10-39510, JP-A-2002-148804, JP-A-2003-248311, JP-A-2005-266075, and JP-A-2006-285193). However, each of the above-mentioned photosensitive resin compositions uses a polyamide acid as a matrix component and hence needs to be cured at a temperature as high as 250° C. or more, which restricts its applications.
Further, photosensitive resin compositions each obtained by applying a specific 1,4-dihydropyridine derivative to an epoxy group-containing compound have been proposed (See JP-A-2003-20339, JP-A-2003-21898, JP-A-2003-48956, and JP-A-2009-167381). However, there has been such a problem that 1,4-dihydropyridine, which is expensive, needs to be incorporated in a large amount in order to provide practical photosensitivity in such a technical field, for example.
Meanwhile, in recent years, there have been increasing demands for increases in capacity of a hard disk drive (hereinafter, also referred to as “HDD”) and in rate of information throughput. Such HDD includes a component called a magnetic head (MRH), which is a thin film, and a component called a suspension board with circuit for supporting the magnetic head. In addition, the suspension board with a circuit to be used in the above-mentioned hard disk drive is a wiring circuit board having a wiring circuit pattern for connecting the magnetic head to a read/write board integrally formed on a suspension board for supporting the magnetic head. That is, the above-mentioned suspension board with circuit is formed so that a base insulating layer made of a polyimide resin is provided on a substrate made of metal such as a stainless-steel foil, a given pattern circuit formed of a conductive layer made of copper is formed as a thin film thereon, a terminal is further formed thereon, and the entire surface excluding the terminal is covered and protected with a cover insulating layer (covering layer), for example (see, for example, JP-A-H10-265572). In addition, the above-mentioned suspension board with the circuit has been widely popularized in recent years because the suspension board with circuit can resist an air flow in the case where the above-mentioned magnetic head and a magnetic disk serving as a recording medium relatively run and retain a minor interval between the suspension board with circuit and the magnetic disk to provide a satisfactory levitation attitude of the magnetic head.
Such a suspension board with a circuit is generally provided with a slider having a magnetic head mounted on the leading end, and there is a need for precisely adjusting a levitation attitude (angle) of the slider relative to a magnetic disk.
In addition, in recent years, in accordance with an improvement in recording density of the magnetic disk, it has been needed to more precisely adjust a pitch static attitude (PSA: attitude angle) of the slider relative to the magnetic disk, and it has been demanded to make a change in PSA due to changes in temperature and humidity as small as possible.
In general, in order to suppress a change in PSA of the slider associated with a change in temperature, coefficients of thermal expansion of a substrate made of metal and a conductive layer are made close to coefficients of thermal expansion of a base insulating layer and a cover insulating layer. Similarly, in order to suppress a change in PSA of the slider associated with a change in humidity, coefficients of hygroscopic expansion of the metal supporting layer and the conductive layer are made close to coefficients of hygroscopic expansion of the base insulating layer and the cover insulating layer.
However, it is extremely difficult to make the coefficients of thermal expansion and the coefficients of hygroscopic expansion of the base insulating layer and the cover insulating layer simultaneously close to those of the substrate made of metal and the conductive layer. In particular, it is very difficult to make the coefficients of hygroscopic expansion of the base insulating layer and the cover insulating layer close to the coefficients of hygroscopic expansion of the substrate made of metal and the conductive layer because the coefficients of hygroscopic expansion of the substrate made of metal and the conductive layer are substantially zero. Thus, under the current circumstances, an attempt to make the coefficients of hygroscopic expansions of the base insulating layer and the cover insulating layer close to those of the substrate made of metal and the conductive layer inevitably sacrifices photosensitivity and other requisite characteristics (see, for example, JP-A-2008-310946).